Replacement for isshiki reports

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Brought up first on discord: most isshiki reports are now dead. It's probably not a good idea to just create new ones in the same place for various reasons, including privilege issues (downvote and flag reports shouldn't be visible to normal users), accessibility and the fact that they were pretty raw outputs.

It would be useful to have this kind of report integrated into the site, something like a /statistics or /reports endpoint.
We currently have the amazing user report by @BrokenEagle, it's been a prototype for three years now and it is a great example of what "official" statistics could look like. Due to the fact that it's processed externally however, that report has been only generated monthly so far. There's also forum limitations because large tables don't scale well visually, on any but the bigger resolutions.

Implementing something similar serverside would have the advantage of being available for shorter time deltas (though weekly, like isshiki was, is well enough), it could be generated much faster, as it would not be bound on api requests, and as already mentioned the access to more "private" reports like up/downvotes, flags and similar could be controlled through already existing mechanisms.

This is related to issue #4217, or rather that issue would be a subset of this if such a thing is implemented.

[nonamethanks]

Fixed by 203067b.